Currently, the type of alarms which are in use in shops and stores to protect products on display are loop alarm systems. In such a system a cable runs from a central alarm unit along the product shelves and back to the alarm unit. Each product is connected to the loop in some manner and there are a number of break points in the loop to enable products to be removed or attached to the loop. If any one break point is disconnected then the alarm sounds and is normally switched off using a keyswitch, thus disabling the whole system. It is then necessary to check each break point in turn and perhaps the whole loop in order to find out which break point has been disconnected.